1. Field of the Invention
This invention relates generally to telecommunication systems, and, more particularly, to wireless telecommunication systems.
2. Description of the Related Art
The list of mobile units that may access wireless communication systems, such as a cellular telephone system and/or a Bluetooth-enabled wireless local area network (WLAN), to transmit voice and data signals has expanded dramatically in recent years to include, among other things, cell phones, personal data assistants, global positioning system receivers, laptop computers, and desktop computers. The types of mobile units on the list, as well as the services they are likely to provide, is expected to continue to grow and the proliferation of wireless communication systems has led users to expect uninterrupted access to these systems and/or networks at virtually any time and in virtually any place.
In part to address this demand for universal access, many mobile units are able to communicate with a variety of different systems and/or networks. For example, a mobile unit may be able to establish a first wireless communication link with a base station in a cellular telephone system that operates according to the Universal Mobile Telephony System (UMTS) protocol while a user is outdoors. The same mobile unit may also be able to establish a second wireless communication link with an access point of a wireless local area network that operates according to the Bluetooth protocol when the user is in a “hot spot” associated with the access point. For another example, a mobile unit may be able to establish a first wireless communication link with a base station in a first cellular telephone system that operates according to the UMTS protocol while a user is in a first cell associated with the first base station. When in a second cell, the same mobile unit may also be able to establish a second wireless communication link with a second base station that operates according to a Global System for Mobile telecommunications (GSM) protocol.
Mobile units typically roam from one system to another, necessitating a handover of the mobile unit. Roaming may include moving from one geographic region to another, from one service provider to another, from one type of wireless communication system to another, from one frequency to another within the same system, and the like. A mobile unit may roam for many reasons. For example, a mobile unit may roam from a first base station to a second base station when a user carries the mobile unit from a first cell associated with the first base station into a second cell associated with the second base station. For another example, a mobile unit may roam from a first frequency to a second frequency when fading of the first frequency reduces the quality of the wireless telecommunications on the first frequency relative to the second frequency.
Wireless telecommunication protocols define messages and measurements that may be used to determine whether a handover is desirable and/or necessary. For example, in the UMTS standard, information related to mobile unit mobility is conveyed to the mobile unit via information elements contained within an RRC MEASUREMENT CONTROL message that conforms to the UMTS RRC protocol standard based on ASN.1 encoding, which is an extensible format for message definitions. The information elements in the RRC MEASUREMENT CONTROL message may indicate that the mobile unit should monitor other UMTS frequencies (e.g. the mobile unit should make one or more inter-frequency measurements), inter-Radio Access Technology types (e.g. the mobile unit should make one or more inter-RAT measurements), and/or the health of the frequency currently being used for wireless communication (e.g. the mobile unit should make one or more intra-frequency measurements). The information elements in the RRC MEASUREMENT CONTROL message defined in the UMTS standard may also be used to determine the types of measurement that will be made, as well as when (e.g. via events) and how often (e.g. via cyclic reporting) the measurement reports are sent back to a Serving Radio Network Controller (SRNC).
Mobile units are typically capable of performing a number of measurements on cells in parallel. The types of cell that can be targeted by the measurements are grouped into three categories: active set cells, i.e. cells which are currently used in soft handover, monitored set cells taken from a neighbor list, i.e. cells that are candidates for the active set, and detected set cells, i.e. cells which are of a different frequency or radio access type. The mobile units use information elements in messages such as the RRC MEASUREMENT CONTROL message to define the attributes of the measurement to be made. For example, in the UMTS standard, the mobile unit determines which measurements to perform using a measurement list stored in an Intra-Frequency Cell Information List within an information element of the RRC MEASUREMENT CONTROL message.
The measurements performed by the mobile unit may be categorized based upon the attributes of the measurement. For example, in the UMTS standard, the measurements performed by the mobile unit may be categorized based upon a frequency and a Common Pilot Channel (CPICH) corresponding to the cell associated with the mobile unit. The CPICH is typically used in UMTS to enable channel estimation. The conventional CPICH uses a predefined bit sequence and has a fixed rate of 30 Kbps with a SF (Spreading Factor) of 256. The CPICH value is typically included in an information element in the RRC MEASUREMENT CONTROL message (e.g. the message transmitted on the downlink) and in an information element in the RRC MEASUREMENT REPORT message (e.g. a report message transmitted on the uplink) when identifying a measurement report to a CPICH. Measurement cells can also be identified via their CPICH value. Furthermore, the desired type of report on the CPICH can be identified by the type of event that is indicated in information elements in the RRC MEASUREMENT CONTROL message.
FIG. 1 conceptually illustrates a path 100 through the information elements of the RRC MEASUREMENT CONTROL message that may be used to set the CPICH value. In the illustrated embodiment and in accordance with the ASIN.1 encoding standard, the path 100 begins at an Intra-Frequency Measurements information element, which includes an Intra-Frequency Cell Information List information element. Information regarding one or more cells is included in a Cell Information information element. A mode associated with the cell may then be chosen. For example, a Frequency Division Duplex (FDD) mode may be chosen, which leads to the Primary CPICH Information information element. Table 1 illustrates a conventional Primary CPICH Information information element.
TABLE 1Primary CPICH (FDD) information elementInformationType andSemanticsElement/Group nameNeedMultireferencedescriptionPrimary scramblingMPInteger(0 . . . 511)codeAs shown in Table 1, the CPICH of a cell typically takes on an integer value in the range 0≦n≦511, where n indicates the primary scrambling code that is used within the measured cell. The value of MP in the “Need” field indicates that this is a mandatory information element.
The events that trigger a measurement report can also be set within information elements in the RRC MEASUREMENT CONTROL message. The UMTS RRC protocol defines a standard set of trigger events. In the interest of clarity, these known definitions will not be repeated herein, however, the trigger events are typically grouped as follows:                1A through 1G—events in FDD that trigger a intra-frequency measurement report;        2A through 2F—events that trigger an inter-frequency measurement report, and        3A through 3D—events that trigger an inter-RAT measurement report.For example, as shown in FIG. 3, under normal UMTS operation the event trigger 1C corresponds to a measurement quality of a CPICH that is not in the active set (CPICH-4) becoming better than a measurement quality of a CPICH of member of the active set (CPICH-3). For another example, the event trigger 1D refers to a measurement quality of CPICH-4 becoming better than a measurement quality of the best CPICH currently available (CPICH-1). Following a trigger event, one or more measurements are transmitted via the RRC MEASUREMENT REPORT message.        
FIG. 2 conceptually illustrates a path 200 through the information elements of the RRC MEASUREMENT REPORT message. In the illustrated embodiment and in accordance with the ASIN.1 encoding standard, the path 200 begins at a Measured Results information element, which includes an Intra-Frequency Measured Results information element. Information regarding one or more cells is included in a Cell Measured Results information element. A mode associated with the cell may then be chosen. For example, a Frequency Division Duplex (FDD) mode may be chosen, which leads to the Primary CPICH Information information element.
Information in the RRC MEASUREMENT REPORT message may then be used to decide whether a handover is desirable or necessary. For example, a mobile unit may provide an RRC MEASUREMENT REPORT message to a Radio Network Controller (RNC). A Radio Resource Manager (RRM) in the Radio Network Controller (RNC) may then use this information to decide whether or not to hand over the mobile unit. The handover is usually performed in a hard or “vertical” manner. There is a complete break in the delivery of data during a hard handover. For example, during a vertical handover from a GSM system to a UMTS system, the call is stopped on the GSM system and re-established on the UMTS system after a short time delay. For another example, during a hard handover from a first frequency to a second frequency of a UMTS system, the call stopped on the first frequency and re-established on the second frequency of the UMTS system. In one embodiment, the inter-RAT measurements and the inter-frequency measurements may be used to determine whether a hard handoff is desirable or necessary.
Conventional wireless telecommunications networks are not capable of performing soft vertical handovers, i.e. a handover in which data is provided substantially continuously, between networks having different radio access types. For example, a conventional Radio Network Controller is not capable of negotiating a soft vertical handover from a UMTS network to a Bluetooth network. The inability of the conventional wireless telecommunications networks to perform soft vertical handovers is due, at least in part, to the absence of a mechanism for detecting the presence of air interfaces formed by disparate networks, as well as a mechanism for measuring and comparing the quality of the communication links formed over the disparate air interfaces.
The present invention is directed to addressing the effects of one or more of the problems set forth above.